The suppression of (neoclassical) tearing modes is of great importance for the success of future fusion reactors like ITER. Electron cyclotron waves can suppress islands, both by driving noninductive current in the island region and by heating the island, causing a perturbation to the Ohmic plasma current. This Letter reports on experiments on the TEXTOR tokamak, investigating the effect of heating, which is usually neglected. The unique set of tools available on TEXTOR, notably the dynamic ergodic divertor to create islands with a fully known driving term, and the electron cyclotron emission imaging diagnostic to provide detailed 2D electron temperature information, enables a detailed study of the suppression process and a comparison with theory. Tearing modes, and, in particular, neoclassical tearing modes (NTMs), have a deleterious effect on the performance and stability of tokamak plasmas. Larger tokamaks, like the proposed ITER tokamak, are more susceptible to the formation of NTMs. It is therefore important to develop techniques to control or suppress them and to gain understanding of the suppression process. Islands can be stabilized by driving a (helical) current perturbation inside the island region. Gyrotrons are an ideal tool for the localized generation of this current through the injection of radio frequency waves into the plasma. This current can either be directly driven noninductively by electron cyclotron current drive (ECCD) [1] or indirectly by heating the island by electron cyclotron resonance heating (ECRH) [2 -4], causing a helical perturbation to the Ohmic current due to the temperature dependence of the plasma conductivity. ECCD is thought to be a more efficient way to suppress (neoclassical) islands. The tearing mode suppression by heating is often neglected. In this Letter, it will be shown that on TEXTOR the physical mechanism at work during heating can be clearly identified. It is demonstrated that also heating gives a sizeable suppression of the islands.A set of tearing mode suppression experiments on the TEXTOR tokamak is described, that focuses on the suppression by heating (ECRH). In TEXTOR, suppression by ECRH dominates over ECCD [5] due to the low current drive efficiency (low T e ).TEXTOR is a medium sized limiter tokamak with a circular plasma cross section (R 0 1:75 m, a 0:46 m) and is ideally suited for island suppression studies due to the unique combination of available tools. With the dynamic ergodic divertor [6], islands can be created and controlled with (in contrast to other tokamaks) a fully known driving term. The gyrotron can be used to generate highly localized EC waves inside the island. Finally, the process of suppression can be observed in detail by the 2D electron cyclotron emission imaging (ECEI) diagnostic [7].The dynamic ergodic divertor (DED) on TEXTOR is a perturbation field experiment consisting of 16 helical coils on the high field side, aligned with the q 3 field lines. Figure 1(a) shows the vacuum field used for the experiments described in this Letter...
High resolution (temporal and spatial), two-dimensional images of electron temperature fluctuations during sawtooth oscillations were employed to study the crash process and heat transfer in magnetically confined toroidal plasmas. The combination of kink and local pressure driven instabilities leads to a small poloidally localized puncture in the magnetic surface at both the low and the high field sides of the poloidal plane. This observation closely resembles the "fingering event" of the ballooning mode model with the high- mode only predicted at the low field side.
Alpha-particle-driven toroidal Alfvén eigenmodes (TAEs) have been observed for the first time in deuterium-tritium (D-T) plasmas on the tokamak fusion test reactor (TFTR). These modes are observed 100-200 ms following the end of neutral beam injection in plasmas with reduced central magnetic shear and elevated central safety factor ͓q͑0͒ . 1͔. Mode activity is localized to the central region of the discharge ͑r͞a , 0.5͒ with magnetic fluctuation levelB Ќ ͞B k ϳ 10 25 and toroidal mode numbers in the range n 2 4, consistent with theoretical calculations of a-TAE stability in TFTR.[S0031-9007 (97)02857-3] PACS numbers: 52.55.Fa, 52.35.Bj, 52.35.Py, 52.55.PiDeuterium-tritium (D-T) plasma operation on the tokamak fusion test reactor (TFTR) provides the first opportunity to investigate the interaction of fusion alpha particles with plasma waves under reactor relevant conditions. Such investigations are crucial for assessing the impact of plasma instabilities on the confinement of energetic alpha particles, which are required to sustain ignition in a D-T reactor. One candidate instability with the potential for affecting alpha particle confinement in tokamaks is the toroidal Alfvén eigenmode (TAE) [1]. This Letter describes the first observation of purely alpha-particledriven TAEs in TFTR with central b a as low as 0.02% (b a ϵ alpha particle pressure͞magnetic pressure), well below that expected in the International Thermonuclear Experimental Reactor (ITER) [central b a ϳ ͑0.5 1͒%].TAEs are discrete frequency modes occurring inside toroidicity induced gaps in the shear Alfvén spectrum which can be destabilized by the pressure gradient of energetic ions. These modes can potentially cause internal redistribution and enhanced loss of energetic alpha particles in a D-T reactor due to their extended radial structure, relatively low instability threshold, and resonant interaction with 3.5 MeV alpha particles near the Alfvén velocity [2,3]. The characteristics of TAEs and associated fast ion losses have been studied in experiments utilizing circulating neutral beam ions ͑E b # 100 keV͒ [4-6], deeply trapped minority ions in the MeV range of energy [7-9], nonlinear beat wave excitation using fast magnetosonic waves [10], and external excitation using saddle coils [11]. Until recently alpha-driven TAEs had not been observed in TFTR, even in the highest fusion power D-T "supershot" plasmas ͑P fus ഠ 10.7 MW͒ with b a ഠ 0.3% in the core of the discharge [12]. These results were consistent with theoretical calculations of alpha-driven TAE stability in TFTR, after taking into account beam ion Landau damping and radiative damping due to coupling to the kinetic Alfvén wave (KAW) [13]. However, a better comparison with theory requires the actual observation of purely alpha particle driven TAEs in D-T plasmas, as described in this Letter. The present experiment was motivated by recent theoretical calculations for low-n modes ͑n , 6͒ in TFTR indicating a significant reduction in the central b a required for destabilizing TAEs under condi...
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